I. Field of the Invention
The present invention relates to metallization techniques which utilize a plasma to treat a surface to which metal will be applied. Particularly, the present invention relates to plasma treatment of the surface of a semiconductor integrated circuit and the subsequent metallization of that surface. More particularly, the technique of the present invention is useful for placing refractory metals, refractory metal nitrides, and refractory metal silicon nitrides on a semiconductor die. A preferred metal for use in the technique of the present invention is tungsten.
II. Background of Related Art
Thin layers of refractory metals are desired for use in integrated circuits for several purposes including without limitation, as low resistance gate interconnections in polysilicon gate regions of field-effect transistors, to form a Schottky-barrier, to form ohmic contacts on silicon, as low resistance vias, and others. As line widths in very large scale integrated circuits (VLSI) decrease, the use of refractory metals in such circuits becomes increasingly desirable.
Traditionally, refractory metals for VLSI applications have been deposited by sputtering, evaporation, and chemical vapor deposition (CVD) onto the active surface of a wafer, or semiconductor substrate, including, without limitation, silicon, gallium arsenide, silicon on sapphire (SOS), silicon on insulator (SOI), silicon on glass (SOG), and other wafers known in the art. Although it is possible to sputter almost any material, including, without limitation, pure refractory metals and refractory metal suicides, sputtering machines tend to be complicated and tend to require considerable maintenance. Sputtering also tends to deposit refractory metals and their suicides in an inconsistent manner which leaves pinholes and other discontinuities in the deposited layer (i.e., sputtered films are not conformal).
Evaporation techniques for depositing refractory metals have many of the same deficiencies as sputtering techniques. Specifically, evaporative deposition of refractory metals is often a complex process. Many techniques which deposit refractory metals by evaporation also provide poor refractory metal coverage.
Relative to sputtering and evaporation techniques, chemical vapor deposition and low pressure chemical vapor deposition (LPCVD) of refractory metals often provide good coverage with reduced system complexity. However, many CVD techniques are somewhat undesirable in that the layers of refractory metals formed thereby are inadequate for some uses in integrated circuits. Many CVD techniques also deposit inconsistent, discontinuous layers of refractory metal. It is also difficult to reproduce the thickness of the refractory metal films produced by some existing CVD techniques.
U.S. Pat. No. 5,540,607, issued to Tsao (the "'607 Patent"), describes a method for treating the surface of a field-effect transistor or a Schottky barrier diode. The technique of the '607 Patent utilizes a plasma etch such as CF.sub.4 +4% O.sub.2 to shape doped polysilicon regions. According to the method of the '607 Patent, the polysilicon surface is then treated with a low power argon plasma to enhance nucleation sites on that surface. A layer of tungsten or molybdenum is then selectively deposited on the treated surface by CVD techniques.
Although the method of the '607 Patent enhances the nucleation sites on a polysilicon or silicon surface, the refractory metal which is subsequently deposited by the reaction of WF.sub.6 with silicon may cause wormholes, which decreases the selectivity of refractory metal deposition. The refractory metal layer may also have an inconsistent thickness (i.e., poor uniformity).
U.S. Pat. No. 5,618,382, issued to Mintz et al. (The "'382 Patent"), describes an apparatus which, among other things, employs a plasma to etch a substrate. The apparatus operates at high frequencies (greater than 13.56 MHz), preferably in the range of 30 to 200 MHz, depending upon the process to be performed on the wafer. The purpose of the apparatus disclosed in that patent is to increase the rate of plasma processing of semiconductor wafers without significant damage thereto. The '382 Patent does not disclose use of the apparatus to deposit refractory metals onto a semiconductor die, nor does the '382 Patent disclose improved adherence of a refractory metal layer to a semiconductor die following plasma surface etching.
In integrated circuit manufacturing, blanket deposition of tungsten is typically favored over selective deposition of the same due to difficulties in controlling selectivity loss. Blanket deposition inherently requires that the tungsten be deposited onto both silicon/polysilicon and oxide surfaces. Typically, prior to the blanket deposition of tungsten, multiple layers, including titanium (Ti) and titanium nitride (TiN), are deposited onto the silicon and/or polysilicon surfaces prior to the deposition of more desirable refractory metals such as tungsten or molybdenum. A first layer, such as a titanium film, which adheres well to silicon, polysilicon, or the like, is deposited directly onto such surfaces. A second layer, such as titanium nitride, which adheres well to both the first layer and to the desired refractory metal, is then deposited onto the first layer. Next, the desired refractory metal, such as titanium or molybdenum, is deposited onto the second layer. The titanium/titanium nitride layers also lower contact resistance and act as a diffusion barrier to prevent fluorine-containing species from attacking the underlying layers during subsequent deposition steps.
Although the use of such layers improves the adhesion of tungsten or other refractory metals to a die, materials such as titanium nitride oxidize very readily under ambient conditions. As those of ordinary skill in the relevant art are aware, refractory metals do not adhere well to oxides. Thus, an oxidized surface of the second layer hinders the ability of the desirable refractory metal to form a contiguous film which will adequately adhere to the second layer (i.e., causes less uniform nucleation and increases the length of time that is required to form a contiguous film).
A method is needed in the art for decreasing or eliminating oxidation on a substrate prior to metallization, so as to improve the adhesion of refractory metals, refractory metal nitrides and refractory metal silicon nitrides to the substrate. A method for producing a more contiguous, uniform layer of refractory metal and in a reduced period of time is also needed. Further, a method is needed which reduces the incidence of spontaneous fluorine attack of the substrate material.